The invention relates to an apparatus for reproducing information recorded at a given write velocity in substantially concentrical tracks on an optically readable record carrier wherein, the information includes an address code as a measure of the radial position on the record carrier. The apparatus comprises:
an optical read device for directing a light beam towards the record carrier, and the light beam is modulated by the information;
a detection device for supplying a signal modulated in conformity with the modulated light beam;
a signal processing device for deriving an output signal and for deriving address codes from the signal of the detection device;
a radial displacer for displacing the light beam in a radial direction across the record carrier, and
a controller for at least controlling the radial displacer, whilst for displacing a position with a known address code to a position with a desired address code the light beam is displaced across a plurality of tracks which is dependent on the desired and the known address codes, the write velocity and the track pitch.
Generally such an apparatus will be an apparatus for optically reading records on which information provided with addresses recorded in a spiral track or in a plurality of concentrical tracks. Such an apparatus may be particularly a Compact Disc Digital Audio disc player or briefly CD player for playing digital audio discs or a Compact Disc Read Only Memory player or simply CD-ROM player for playing digital data discs.
In addition to the actual audio information so-called sub-code information is also recorded on a digital audio disc wherein the sub-code information includes, inter alia an absolute time code expressed in minutes, seconds and frames (1/75 s) which indicates the time elapsed from the start of playing the disc.
This time code is used as an address code in order to bring the spot of the light beam to a desired radial position on the disc. In most commercially available CD-players, this address code is exclusively used for finding the beginning of the music tracks on the disc. In fact, the absolute time codes of the beginning of the music tracks present on the record, expressed in minutes and seconds, are indicated on the lead-in track of an audio disc. Since seconds are not sufficiently accurate to find the beginning of a music track, use is made of the relative time code also incorporated in the sub-code signal and indicating the time elapsed from the beginning of each music track. In addition to searching for the beginning of music tracks by using the absolute time code, some players have a facility to directly program an arbitrary absolute time code so that the spot of the light beam can be moved to any arbitrary position on the disc and any arbitrary music track can be played.
In both cases the desired position is achieved in the same manner. The number of tracks across which the spot of the light beam must be displaced to reach the desired position is calculated from the absolute time code of the beginning of a desired music track and the absolute time code of the position from which a jump must be made. The comparison indicating the relation between the number of tracks to be jumped and the absolute time code is dependent on the track pitch and the velocity with which the information has been recorded on in particular disc. In accordance with the worldwide adopted Compact Disc Standard, this write velocity must be between 1.2 and 1.4 m/s and the track pitch must be between 1.5 and 1.7 .mu.m. The actual write velocity and track pitch of an arbitrary disc to be played are, however, not known. In order to prevent the spot of the light beam from overshooting the desired position and, in the worst case, from overshooting the lead-out track of the disc, the minimum write velocity and the maximum pitch are used as a basis for the calculation of the number of tracks to be jumped.
However, the result thereof is that jumping to desired address is not very accurate so that in most cases a number of jumps must be made in order to reach the desired address. After one jump the light beam may still be situated 800 to 1000 tracks before the track with the desired absolute time code.
The absolute time code on a CD-ROM disc is not only present in the sub-code signal but also at the start of each data block. The various data blocks can now be read by searching the relevant absolute time code in the data. The Philips CD-ROM player CM 100 incorporates a table stating the associated track number for each minute. The number of tracks to be jumped from a given address to the address of the data block to be read is determined by means of this table. In order to prevent overshoot of the desired position, the table stored in the player is based on the minimum write velocity and the maximum track pitch. This means that also in a CD-ROM player jumping to a desired position is not very accurate so that the searching time is relatively long.